The invention lies in the field of optical transmission systems operating at a single wavelength or with wavelength division multiplexing (WDM). The invention relates more particularly to amplifying the optical signals conveyed in such systems.
In general, amplitude-modulated optical signals for transmission purposes are amplified by means of fiber amplifiers such as erbium-doped fiber amplifiers since they do not present gain non-linearity as a function of input signal power at the modulation frequencies (of the order of 100 MHz to 10 GHz) of the signals used. Although such amplifiers do indeed present a phenomenon of saturation gain, when transmitting binary signals at several gigabits per second, the lifetime of the carriers is long enough relative to the bit time to ensure that gain remains insensitive to the fluctuations in light power that correspond to the binary modulation.
However, with semiconductor light amplifiers, the carrier lifetime is short enough so that under saturation conditions gain varies with the modulation rate. This degrades the extinction ratio and corresponds to compressing gain. However if it is desired to conserve a linear response, then input light power must be limited, which is unfavorable for signal-to-noise ratio.
This problem arises in particular when amplifying a single wavelength signal. With a WDM signal, the instantaneous light power injected into the amplifier is the sum of the powers of the channels constituting the signal and the pulses in a plurality of channels are superposed in varying numbers at each instant, but often with random phases. This can give rise to strong temporary variations at high frequency in the total power of the signal, but these frequencies are generally filtered by the amplifier. However, if the pulses are in phase, then the problem becomes highly critical. After amplification and spectrum demultiplexing, the pulses in each channel present extinction ratios that are degraded and variable from one pulse to another, which harms the quality with which level detection takes place at the receiver.
The problem of gain saturation in semiconductor light amplifiers also arises when they are used as optical gates in switching systems for optical networks. In order to accommodate high data rates, it is necessary to have a good signal-to-noise ratio. That requires the signals injected into the gates to be at high power, but under such circumstances, their extinction ratio is degraded.
Thus, in spite of the cost and compactness advantages of semiconductor amplifiers, their field of use is limited.
To resolve this problem, a first approach consists in seeking to increase the saturation power thereof by optimizing their dimensioning and the composition of the semiconductor layers making them up, or by adopting active structures that are complex, e.g. multiple quantum wells.
Another approach consists in using an amplifier whose gain is stabilized, e.g. as described in European patent application EP-A-0 639 876 published on Feb. 22, 1995. That solution does indeed enable the linear operating range of the amplifier to be extended, but it presents the drawback of a limited dynamic range in terms of power, which is particularly constricting with WDM signals. In addition, the gain of stabilized-gain amplifiers is set on manufacture, thus reducing flexibility of the amplifier faced with the variety of applications and contexts of use.
An object of the invention is to avoid the limitations of the methods mentioned above. For this purpose, the invention proposes a solution using external means that enable conventional type semiconductor amplifiers to be used operating under gain saturation conditions while also providing gain that is constant for the modulated signals that are to be amplified.
More precisely, the invention provides a device for using a semiconductor light amplifier to amplify an amplitude-modulated optical signal referred to as the signal to be amplified and carried by at least one signal wavelength, the device comprising:
compensation means for producing a compensation light wave; and
coupling means for injecting said optical signal to be amplified and said compensation light wave into said amplifier;
and wherein said compensation means comprise modulation means that are controlled directly as a function of the modulation of the signal to be amplified so as to produce said compensation light wave by amplitude modulating at least one auxiliary light carrier wave in such a manner that the combination of said optical signal to be amplified and of said compensation light wave has amplitude modulation that is suppressed or at least attenuated.
To ensure there is no modulation of the total light power injected into the amplifier, it would be necessary for the compensation wave generated by the modulation means to be modulated inversely relative to the modulation of the optical signal to be amplified and for the modulation amplitudes of the signal and of the compensation wave to be equal in absolute value at all times. However, even if those conditions are not satisfied in full, a significant improvement in the linearity of amplification is still obtained.
According to an additional characteristic of the invention, said auxiliary light carrier wave has a compensation wavelength that is different from said signal wavelength(s). This disposition is necessary when the compensation wave and the optical signal to be amplified are codirectional, i.e. when they propagate in the same direction within the amplifier, since it must be possible subsequently to eliminate the compensation wave by filtering. In the opposite case, this disposition is nevertheless desirable in practice in order to ensure that beat noise between the signal and the compensation wave is avoided.
In numerous applications, it is necessary to amplify a signal that is received in optical form. The signal as received or a sample taken from said signal constitutes an input signal to the device, and the modulation means must then transfer the inverse of its modulation (i.e. in phase opposition) onto the compensation wavelength. Furthermore, the compensation wave and the signal to be amplified, i.e. the signals together making up said input signal injected into the amplifier, must be modulated in properly synchronous manner.
In addition, in the device of the invention, said compensation means include a delay device supplying said signal to be amplified from an input optical signal, and said modulation means are suitable for modulating the amplitude of said auxiliary light carrier wave inversely relative to the modulation of said input signal.
To attenuate modulation of the total light power injected into the amplifier to a significant extent, it is appropriate for the modulation means to present good linearity relative to the complement of the input signal. This object can be easily achieved using optoelectronic conversion means followed by electrooptical conversion means.
Thus, in a first possible embodiment, the modulation means comprise a photodetector receiving a portion of the input optical signal and controlling an electrooptical modulator. The compensation light wave is then the result of the auxiliary wave being modulated by means of the electrooptical modulator.
Whatever the electrooptical modulator that is selected, it is always possible to obtain a response that is sufficiently linear by adjusting the applied control and/or by limiting the excursion of said control. Furthermore, by adjusting the power level of the auxiliary wave, it is possible to equalize the modulation amplitudes of the signal to be amplified and of the compensation wave.
The electrooptical modulator can have an interfero-meter structure or it can be an electroabsorption modulator. Electroabsorption can be preferable to achieve a linear response over a wider range of modulation amplitudes. In a variant, the electrooptical modulator and the laser source are constituted by a laser with an integrated modulator.
The modulation means can also be implemented in fully optical manner. At very high data rates, this solution presents the advantage of avoiding the need to develop electronic circuits capable of operating at such high frequencies.
Thus, in a second possible embodiment, the modulation means comprise a second semiconductor light amplifier receiving said auxiliary light carrier wave and a fraction of said input optical signal, said compensation light wave being the result of amplifying said auxiliary wave by means of said second amplifier whose gain is modulated by said fraction of the input optical signal.
In a variant, the modulation means comprise an interferometer structure provided with two branches, each having first and second ends, said first ends receiving respective fractions of said auxiliary wave, one of said branches including a material whose index varies as a function of the light power received and receiving a fraction of said input optical signal, said second ends being coupled together to provide said compensation light wave.
The invention can also apply to cases when the optical signal to be amplified is produced from an input signal available in electrical form. The modulation means can then be used directly on said input electrical signal.
In which case, said modulation means are constituted by an electrooptical modulator receiving said auxiliary wave and controlled by an electrical signal complementary to said input electrical signal.
Advantageously, the semiconductor light amplifier has a cutoff frequency that is lower than that of said modulation means.